1. J. M. Jin, W. F. Lin and P. A. Christensen
Exploring the possibility of COads oxidation at Ru(0001) and Ru/pc-Pt electrodes at open circuit, and the potential role of active O species at Ru, using in-situ FTIR spectroscopy
J. M. Jin, W. F. Lin and P. A. Christensen
School of Chemical Engineering and Advanced Materials,
University of Newcastle upon Tyne,

2. Winshields Crag on the Roman Wall, GOC

3. Direct/Indirect Methanol Fuel Cell
Why Ru?
Direct/Indirect Methanol Fuel Cell
CH3OH + Pt(s) Pt-CH2OH + H+ + e-
Pt-CH2OH + Pt(s) Pt2CHOH + H+ + e-
Pt-CH2OH + Pt(s) Pt3COH + H+ + e-
Pt3COH Pt-CO + 2Pt(s) + H+ + e-
Pt(s) + H2O PtOH + H+ + e-
Pt-CO + PtOH Pt(s) + CO2 + H+
Pt-CO + RuOH Pt(s) + Ru(s) + CO2 + H+ CO L

4. Community content with Langmuir-Hinshelwood inspired Bifunctionality mechanism of Watanabe and Motoo (1975):
Pt-CO + Ru-OH* Pt + Ru + CO2 + H+ + e-
ie “CO-tolerant anode”
Surface electrochemistry of Ru not really investigated until recently, despite importance in direct and indirect methanol fuel cell electrocatalysis.

5. However:
Watanabe and Motoo also postulated:
Pt-CO + Ru-O* Pt + Ru + CO2
Pt-CO + 2RuOH Pt + 2Ru + CO2 + H2O
And:
No-one, until recently, considered CO surface mobility.

6. Summary of electrochemical data

7. Remember- Ru just supplies ‘active oxygen’…..

8. Cyclic voltammogram of the Ru(0001) in 0.1 M HClO4. Sweep rate 50mV/s.
Lin et al, J. Phys Chem. B 104 (2000) 6642
Lin et al, J. Phys. Chem. B 104 (2000) 12002

9. At the gas-solid interface, Ru(0001) is inactive towards the oxidation of adsorbed CO: the Ru-O bond of the (2 x 2)-O layer formed at low oxygen coverage is too strong, and the Ru-O bond of the (1 x 1)-O layer formed at high coverages remains strong, and the coverage is such that CO is prevented from adsorbing anyway.
It is not clear what the oxygen-containing layer at Ru(0001) is when immersed in aqueous electrolyte, whether –O or –OH. The (2 x 2)-O(H) is still inactive, but the (1 x 1)-O(H) layer, formed at potentials > 200 mV, is active, reaction with COads taking place at the perimeter of the (1 x 1)-O(H) domains. We have also shown that the frequency of the COads is an effective probe of the structure of the surface.

10. Ru(1 x 1)-O(H) active towards oxidation of COads
What is the mechanism?
RuOH + Ru-CO Ru + CO2 + H+ + e-
RuO + Ru-CO Ru + CO2
2RuOH + Ru-CO Ru + CO2 + H2O
Watanabe and Motoo, 1975

11. In-situ FTIR studies on COads oxidation at Ru(0001) at OCP

12. The in-situ FTIR experiments were carried out in N2-sat
The in-situ FTIR experiments were carried out in N2-sat. 0.1M HClO4 unless otherwise stated.
Experiments at 20 C and 55 C, only 20 C data presented here.

13. Electrode preparation
In Newcastle, the Ru(0001) single crystal electrode (Fritz-Haber Institute) was polished with mm alumina, washed thoroughly with millipore water and ultra-sonicated in millipore water for a few minutes prior to transfer to the cell.
The polycrystalline Pt electrode was ‘top hat’–shaped with an area of 0.64 cm2 exposed to the electrolyte. The Ruad/Pt electrode was prepared by the spontaneous deposition of Ru onto the pc-Pt electrode from a solution of 0.2M RuCl3/0.1M HClO4 for 1 minute; the coverage of Ru was determined according to the method of Watanabe and Motoo and found to be c. 0.3.

14. Solution sparged with N2
15 min
CO2 formation
at open circuit as a
function of time.
CO pre-adsorbed on the
Ru(0001) at -200 mV in
0.1 M HClO4.
Background at -200 mV,
then open circuit.
Solution sparged with N2
2340
10 s

15. COL
COH
COads Spectra at open circuit as a function of time. Reference spectrum collected at 1100 mV.
100 co-added and averaged scans at 8 cm-1; 16s.

16. O O C C
COL
COH

17. ?
50s
Plot of the IR band intensity of the COL at open circuit as a function of time. (CO2 intensity is also shown).

18. RuCOL dn/dE = 30 cm-1/V
Plot of the frequency of the COL feature at open circuit as a function of time.

19. //bm 550 mV:
At 50 C, CO2
evolution
commenced
once OCP rose
above 125 mV
Open circuit potential estimated from the frequency of the COL feature as a function of time.

20. The induction time is required for OCP to increase to +200 mV in order to initiate the growth of the active (1 x 1)-O phase which
is required before oxidation of COads can
take place

21. Ru(0001)
Very little change
in OCP (see next
slide) from 500 mV
CO2 intensity as a function of time on holding the COads/Ru(0001) electrode at the //bm potential of +550 mV for 50s prior to switching
to open circuit.

22. Plot of the frequency of the COL band as a function of time
OCP changes little,
rate of COads oxidation
unchanged on switching
to OCP
Plot of the frequency of the COL band as a function of time

23. Conclusions
The oxidation of adsorbed CO to CO2 takes place at OCP at Ru(0001) in 0.1M HClO4 via a chemical process. (First observation of this!)
If the potential of the Ru(0001) electrode is stepped to the equilibrium OCP value and held, and then the circuit is broken, the rate of evolution of CO2 remains unchanged.
The oxidation of COads only takes place once the OCP rises into the region where the active (1x1)-O oxide is formed.
The rate of oxidation of the COads to CO2 is highly dependent upon the temperature, anion, etc.

24. COads oxidation at Ru/pc-Pt at OCP

25. qRu = 0.3
In-situ FTIR spectra
taken from the Ru/pc-Pt electrode immersed in 0.1M HClO4 at open circuit after CO adsorption at -200 mV vs Ag/AgCl. Ref. Pot mV.
Friedrich et. al.,
J. Electroanal. Chem., 524 (2002) 261
CO on Ru/Pt(111) Ru Pt
Polycrystalline Pt!

26. COL and CO2 intensities, and OCP in 0.1M HClO4
Ru/pc-Pt
COL and CO2 intensities, and OCP in 0.1M HClO4

27. Re-orientation of off-normal COL
The initial rapid rise in the OCP is not observed in the absence of adsorbed CO.
There is marked rise in the intensity of the COL bands (c. 16% in the case of the COL,Ru).
Re-orientation of off-normal COL

28. cff Hayden *
Possible COads stripping routes

29. The work of Koper et al, Far. Diss., 121 (2002) 285
Corrugation potential for CO adsorption on the Ru-modified Pt(111) surface and the different diffusion hops considered in the model, from perodic Density Functional Theory calculations. No diffusion is allowed from the Ru to the Pt because of the very high activation barrier involved.

30. One of the ‘Two-zone models’ for CO oxidation at Ru/Pt(111)
OH + CO
Zone 2 CO CO Ru Ru Ru Ru Pt Pt
Pt* Pt Pt Pt Pt
Pt* Pt Pt Pt
One of the ‘Two-zone models’ for CO oxidation at Ru/Pt(111)

31. COL and CO2 intensities, and OCP in 0.1M HClO4
Ru/pc-Pt
Free interchange of COads between Ru & Pt sites
Onset of
(1 x 1)-O
COL and CO2 intensities, and OCP in 0.1M HClO4

32. COL frequency and OCP in 0.1M HClO4
Ru/pc-Pt
COL frequency and OCP in 0.1M HClO4

33. CO2 frequency and OCP in 0.1M HClO4
Ru/pc-Pt
Markedly different to COL Ru data!!
CO2 frequency and OCP in 0.1M HClO4

34. Conclusions
Under open circuit conditions:
COL,Ru and COL,Pt can be oxidised by O-containing adlayer on Ru to CO2.
Free exchange of COL between the Ru and Pt sites.
Ru-CO + Pt* ↔ Ru + Pt*-CO
Pt-CO + Pt* ↔ Pt + Pt*-CO

35. The initial potential-determining species is a small amount of adsorbed hydrogen.
During the rapid stripping of this small amount of adsorbed H, the OCP rises dramatically and the COL species on the Pt and Ru re-orientate to the vertical.
Once the hydrogen is stripped, oxygen is chemisorbed at the Ru edges of the Ru islands adjacent to the Pt* sites and possibly also at the Pt* sites. The adsorbed oxygen drives up the potential, causing the formation of the relatively inactive (2 x 2)-O(H) layer at potentials up to 200 mV. COL species on the Pt and Ru are oxidised through diffusion to the Pt* sites and reaction with adjacent Ru-OH.

36. E > 200 mV active (1 x 1)-OH layer forms leading to more rapid oxidation of COL ON THE Ru.
Initially, the formation of the
(1 x 1)-OH compresses the COL left on the Ru, leading to increased local coverage. Eventually, these domains break up.
Over longer time scales, all the COL are oxidized, the Ru sites are fully covered by the (1 x 1)-O(H) layer and the potential rises to its equilibrium value.

37. See: J. M. Jin, W. F. Lin and P. A. Christensen,
J. Electroanal. Chem., 563 (2004) 71.
P. A. Christensen, J-M. Jin, W-F. Lin, and
A. Hamnett, J. Phys. Chem. B, 108 (2004) 3391